No full partner scan for default recipients

Commit message

[PERF] mail: skip OdooBot partner count for default recipients

_message_get_default_recipients counted partners sharing OdooBot's email
on every call to decide whether to ban it. res.partner.email_normalized
is not indexed, so this is a full table scan, done e.g. once per invoice
in the send wizard. Only run it when OdooBot's email is a candidate,
and stop at the first match.

Also add a btree index on res.partner.email_normalized: exact-match
searches on it are common (find_or_create, recipient lookups).

Benchmark: `_message_get_default_recipients` on one partner, 667k
partners (median of 10 runs after warmup):

| Version | SQL | ms |
| --- | ---: | ---: |
| Baseline | 5 | 97.6 |
| PR | 3 | 1.0 |

The skipped query alone takes ~250 ms on a 2M partners table.